1. Field of the Invention
This invention relates to an insulated gate field effect semiconductor device using a thin film semiconductor and a method for producing the same.
2. Description of the Related Art
A known structure of an insulated gate field effect transistor using a thin film semiconductor (hereinafter referred to simply as "TFT") is shown in FIG. 2D. The method for producing such TFT is described below with reference to FIGS. 2A to 2D.
A base film (silicon oxide film) 22 is formed on a glass substrate 21 at a thickness of approximately 2000 .ANG.. On the base film 22, a silicon semiconductor layer 23 having amorphous or crystalline structure is formed as an active layer (where source/drain regions and a channel forming region are formed) at a thickness of about 1000 .ANG. to obtain the shape shown in FIG. 2A. An element separation patterning is performed to obtain a shape shown in FIG. 2B. During this patterning, it is difficult to etch only the active layer 23, and the base film 22 is also etched to some extent. As a result, a recessed portion 24 occurs on the base film 22.
A silicon oxide film 26 as the gate insulating film is formed at a thickness of approximately 1000 .ANG.. As seen in FIG. 2C, however, the film 26 also generates a recessed portion 27. FIG. 4 is a cross section TEM photograph corresponding to the shape shown in FIG. 2C. The photograph represents the state of thin film at the recessed portion 27 where a concave strip appears to form a notch.
After this etching, an aluminum film 28 is formed at a thickness of 6000 .ANG., and the film 28 is patterned to form a gate electrode. Then an anodizing treatment is given to the patterned electrode to form an oxide layer 29 at a thickness of 2000 .ANG.. FIG. 2D shows the A-A' cross section of FIG. 2C. As illustrated in FIG. 2D, the aluminum film 28 is patterned to form the gate electrode. FIG. 3 is a schematic drawing of a plan view of a TFT shown in FIG. 2C or FIG. 2D. The C-C' cross section of FIG. 3 corresponds to FIG. 2D, and the B-B' cross section corresponds to FIG. 2C. The reference numbers 30 through 32 in FIG. 3 are the contact electrodes, though they are not shown in FIG. 2C and FIG. 2D.
A problem of such TFT is that the presence of recessed portion 27 causes substantial break of the gate electrode and the gate wiring 28. The breaking is presumably caused by the following phenomena.
1. The patterning of the gate electrode 28 made of aluminum is preferably conducted by a selective etching using a wetetching method. By this etching process, however, an etchant solution enters into the recessed portion 27. As a result, the recess is enlarged, and, in the worst case, the gate electrode 28 breaks at the portion 34. PA0 2. By anodizing after the aluminum film 28 is patterned, the surface of the patterned gate electrode 28 is oxidized. During the anodizing, however, the electrolyte solution enters into the recessed portion 27 to oxidize the portion 34 from the gate electrode side. Consequently, the gate electrode 28 increases its resistance and further becomes insulated.
The defects of TFT are supposed often to occur by the combination of these reasons. The production of TFTs having a structure shown in FIGS. 2C to 2D faces reduction of a yield.